1. Field of the Invention
The present invention relates to microcapsules and methods of microencapsulating a core of fill material. The resulting microcapsules are adaptable to a variety of applications, but particularly for use in carbonless copying systems.
2. Background of the Invention
Microcapsules generally comprise a core of fill material surrounded by a wall or shell of polymeric material. The fill material may be either gaseous, liquid, or solid, and may be composed of a single substance, a solution, a suspension or a mixture of substances. The wall surrounding the core of fill material acts to isolate the fill material from the external environment. When it is desirable to release the fill material, the capsule wall may be ruptured by mechanical pressure, for example, thereby introducing the fill material into its surroundings. Generally, microcapsules comprise separate and discrete capsules having non-interconnecting hollow spaces for a fill material. The fill material is thus enveloped within the generally continuous polymeric walls of the microcapsules, which may range from 0.1 to approximately 500 microns in diameter.
Uses for microcapsules are as varied as the materials that can be microencapsulated. Of particular importance are the uses of microcapsules in medicinal and biological preparations, fertilizers, flavorings, deodorizers, adhesives, xerographic toners, and carbonless copying systems.
Though microcapsules and microencapsulation techniques are applicable to a wide variety of products, one of the most significant applications is their use in carbonless copying systems. The present invention is particularly adaptable to carbonless copying systems and will be discussed primarily in connection with such systems. However, it should be understood that the invention is not limited to carbonless copy applications and may be used wherever the use of microcapsules is beneficial.
Carbonless copying systems usually include a plurality of paper sheets arranged in a manifold set, each sheet of the set having one or more coatings on its surfaces. The manifold set is designed so that when a marking pressure caused by a typewriter, pen, or other instrument is applied to the outermost sheet, a colored mark will be formed on at least one surface of each sheet of the manifold set.
To this end, the top sheet of the manifold set to which the marking pressure is applied is provided with a coating on its back surface. This coated back surface includes microcapsules containing an initially colorless chemically reactive color-forming dye precursor as the fill material. The upper surface of the next sheet, which is adjacent to the back surface of the top sheet, is coated with a material containing a component, such as phenolic resin or reactive clay, that is capable of reacting with the colorless dye precursor contained in the microcapsules to produce a color. Thus, a marking pressure on the upper surface of the top sheet will rupture the microcapsules on the bottom surface and release the colorless dye precursor. The colorless dye precursor then chemically reacts with the reactive component of the coated front of the lower sheet to produce a colored mark corresponding to the area of marking pressure. In similar fashion, colored marks are produced on each succeeding sheet of the manifold set by the marking pressure rupturing the microcapsules carried on the lower surface of each sheet.
The sheets of the manifold set in carbonless copying systems are designated in the art by the terms CB, CFB, and CF, which stand respectively for "coated back", "coated front and back", and "coated front". The CB sheet is usually the top sheet of the manifold set and the sheet upon which the marking pressure is applied. The CFB sheets are the intermediate sheets of the manifold set, each of which is able to have a mark formed on its front surface by a marking pressure and each of which also transmits the contents of ruptured microcapsules from its back surface to the front surface of the next sheet. The CF sheet is the bottom sheet and is only coated on its front surface so that an image may be formed on it.
While it is customary to have the coating containing the microcapsules on the back surface of the sheets and to have the coating containing the reactive component for the capsules on the front surface of each of the sheets, a reverse arrangement is also possible. In addition, one or more of the reactive ingredients may be carried in the sheets themselves, rather than applied as surface coatings. Furthermore, the reactive component for the colorless dye precursor may be microencapsulated instead of or in addition to the precursor. Patents illustrative of the various kinds of systems that may be used in the production of manifold carbonless copying systems include by way of example: U.S. Pat. Nos. 2,299,694 (Green); 2,712,507 (Green); 3,016,308 (Macaulay); 3,429,827 (Ruus); and 3,720,534 (Macaulay et al).
The literature also contains many methods and techniques for preparing microcapsules, whereby two or more reactive components are brought together to form a microcapsular wall. A majority of these methods form the encapsulating walls by providing minute discrete droplets containing the intended fill material dispersed within a continuous phase that contains at least one of the reactive components. In one class of microencapsulation technique, the walls of the microcapsules are formed from reactive components that are present only in the continuous phase and not within the dispersed droplets. Examples of such microencapsulation methods are the urea-formaldehyde polymerization technique disclosed in U.S. Pat. No. 3,016,309 (Macaulay), the coacervation methods described in U.S. Pat. No. Re. 24,899 (Green) and the in situ polymerization method taught in U.S. Pat. No. 3,219,476 (Robbins).
The Macaulay patent teaches the formation of a high molecular weight urea-formaldehyde condensate wall from a urea-formaldehyde precondensate that is present in the continuous phase. The reaction is carried out by adjusting the pH of the continuous phase. The Green patent discloses forming a gelatinous coating around oil droplets containing the fill material. This coating is then hardened into microcapsule walls by cross-linking agents present in the continuous phase. The Robbins patent describes a method of encapsulating aerosol particles. Aerosol particles are first formed followed by intimately contacting said particles with an unsaturated organic monomer selected from the group consisting of diolefins, vinyl esters, or esters of, B-unsaturated acids having from 4 to 25 carbon atoms and causing said monomers to polymerize on said aerosol particles thereby encapsulating the particles.
A second class of microencapsulation is interfacial polycondensation exemplified by U.S. Pat. No. 3,429,827 (Ruus). The method taught by Ruus includes producing an aqueous dispersion of a water immiscible organic liquid containing one of the reactive components. A second reactant is then added to the aqueous phase whereupon the reactants form a polymer wall at the interface between the aqueous and organic phases.
A third class of microencapsulation is in-situ polymerization. In this class of microencapsulation techniques, the microcapsule walls are formed from materials present only in the discontinuous phase. Thus, the wall forming materials dispersed into the discontinuous phase polymerize and migrate outward to the interface between the discontinuous and continuous phases, resulting in the formation of the microcapsule wall. Known techniques of in-situ polymerization include free radical polymerization and the incorporation of reactive polyisocyanates and polyol compounds within the discontinuous phase.
A wide variety of microencapsulation methods are known in the art as exemplified by the following patents. U.S. Pat. No. 4,187,194 (Wellman et al) discloses a microencapsulation process directed primarily to the encapsulation of xerographic toner particles. The process calls for altering the solubility characteristics of the solvent that contains the wall material and the core material in order to effect phase separation. The phase separation is accomplished by vaporizing the solvent or by adding a non-solvent. U.S. Pat. No. 4,016,099 (Wellman et al) discloses a similar process, except that phase separation is effected by the addition of a second solvent that is miscible with the first solvent, but in which the wall material is substantially insoluble. Neither of the Wellman, et al. patents discloses the use of amines or epoxy resins as wall materials.
U.S. Pat. No. 3,928,230 (Unsworth et al) discloses a microencapsulation process whereby the microcapsule walls are formed from epoxy resin monomers and polyamino compounds. The disclosed process involves dissolving the epoxy monomers in an organic solvent and dispersing the solution in water. The substance to be encapsulated is then likewise dispersed in water and an organic solution of the polyamino compound is then mixed into the emulsion. This is not an in-situ polymerization process.
U.S. Pat. No. 3,726,804 (Matsukawa et al) describes a process of microencapsulation by in-situ polymerization. The two wall forming materials are dissolved in an oily liquid core material in the presence of a low-boiling solvent or a polar solvent. After emulsification, the temperature of the system is raised, such that the low-boiling or polar solvent is released from the oily liquid and the wall forming materials are transferred to the surface of the dispersed oil drops, where polymerization and microencapsulation occur. U.S. Pat. No. 3,822,181 (Vassiliades et al) discloses a method of making air-containing microcapsules for holding pigment. U.S. Pat. No. 4,140,336 (Maalouf et al) discloses the treatment of existing, pre-formed polyamide capsules with epoxy resin, preferably a bisphenol A based epoxy resin. The pre-formed polyamide capsules are preferably made by interfacial polycondensation.
In the context of carbonless copying systems, one common problem is the tendency for color to inadvertently develop on the CF and/or CB sheets. This unwanted color development is usually due to the presence of free colorless dye precursor on the CB sheet. The presence of free precursor on the CB sheet may be due to incomplete microencapsulation or accidental microcapsule rupture. Additionally, the walls of produced microcapsules containing the precursor may include faults or pores that allow the colorless dye precursor to leak from the microcapsules. This free dye precursor often causes discoloration by contacting the reactive CF component and thereby changing the dye precursor to its colored form. Discoloration, which has been variously referred to as blush, offset, bluing, ghosting, backprint, etc., is highly objectionable and undesirable in carbonless copying systems. Thus, there is a need for microcapsules that are relatively impermeable to dye precursor solutions such that unacceptable levels of discoloration are avoided.